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Subelement A
VHF-DSC Equipment & Operation:
Section 4
Receiver Faults-1:
What condition may prevent a VHF radio receiver from staying tuned to the desired channel?
  • The local oscillator frequency is beating against the incoming signal to produce an Intermediate Frequency (IF).
  • Correct Answer
    The output signal of the Phase Locked Loop (PLL) circuit in the frequency synthesizer has become unlocked.
  • The discriminator circuit is alternately developing positive and negative voltages at the modulation frequency.
  • The ratio detector circuit is producing a variable output that deviates across the receiver's dynamic range.
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What fault may exist if a VHF radio receiver fails to produce audible output from the speaker and the squelch control has no effect?
  • A component in the RF amplifier stage has failed or the signal from a preceding stage is grounded.
  • The Local Oscillator stage is no longer supplying a signal to the mixer stage at the correct frequency.
  • Correct Answer
    The AF amplifier may be defective or the transmitter Push-To-Talk circuit may be activated.
  • The mixer stage is no longer producing an output signal on the correct Intermediate Frequency (IF).
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What is indicated if a VHF radio fails to receive nearby stations and a voltmeter connected between the positive side of a radio's DC line and ground reads zero line voltage? The fuse in the VHF radio also reads zero when checked with an ohmmeter.
  • The antenna may be defective or water intrusion has affected antenna system performance.
  • The coaxial transmission line may be open, shorted, or a connector has become intermittent.
  • A fault in the discriminator circuit is producing zero output voltage during peak modulation.
  • Correct Answer
    A "crowbar" circuit has prevented an over current condition that might damage the radio.
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What fault may exist if a VHF radio receiver fails to produce audible output from the speaker and the squelch control has no effect?
  • Correct Answer
    The AF amplifier may be defective or the transmitter Push-To-Talk circuit may be activated.
  • A component in the RF amplifier stage has failed or the signal from a preceding stage is grounded.
  • The Local Oscillator stage is no longer supplying a signal to the mixer stage at the correct frequency.
  • The mixer stage is no longer producing an output signal on the correct Intermediate Frequency (IF).
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 What condition may prevent a VHF radio receiver from receiving a weak signal?
  • The local oscillator frequency is beating against the incoming signal to produce an Intermediate Frequency (IF).
  • The output signal of the Phase Locked Loop (PLL) circuit in the frequency synthesizer has become unlocked.
  • The output signal of an AF amplifier stage is greater than the input signal.
  • Correct Answer
    The weak RF signal has been suppressed at the limiter stage of the received by capture effect.
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Which of the following is the best test procedure to isolate the source of distortion in the audio output of a receiver?
  • Correct Answer
    Use an oscilloscope to visually display AC signals on the output of each AF amplifier stage.
  • Use a spectrum analyzer to visually display the amplitude and frequency of each AF amplifier stage input.
  • Use a signal generator to inject a 1 KHz signal into the first AF amplifier stage and evaluate audio output.
  • Use a digital voltmeter to measure transistor bias voltages in each AF amplifier stage.
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